1. Field of the Invention
The present invention generally relates to a fingerprint image forming apparatus, a finger movement amount calculation method, and a fingerprint image forming method. More particularly, this invention relates to a fingerprint image forming apparatus including two line sensors, and a finger movement amount calculation method and a fingerprint image forming method used for the fingerprint image forming apparatus.
2. Description of the Related Art
There is a conventional fingerprint reading system that includes a sensor having a surface area smaller than that of a finger. Such a fingerprint reading system reads a fingerprint of a finger moving over the sensor portion by portion and reconstructs the image of the fingerprint from the read partial images (see, for example, patent document 1). A sensor that reads a fingerprint portion by portion is generally smaller than a two-dimensional sensor that can obtain an entire image of a fingerprint in one scan. Using such a small sensor makes it possible to reduce the size and costs of a fingerprint reading apparatus.
A fingerprint reading system disclosed in patent document 1 includes a line sensor for reading a fingerprint line by line and is configured to reconstruct the image of a fingerprint from lines of image data. One problem with a fingerprint reading system using a line sensor is that overlaps between lines of image data vary depending on the speed of a finger moving over the line sensor. In other words, if an image of a fingerprint is reconstructed by simply accumulating lines of image data, the length of the reconstructed image varies depending on the movement speed of the finger. Such variation in the length of a fingerprint image, for example, reduces the reliability of a personal identification system using fingerprints. Therefore, when forming a fingerprint image using a line sensor, it is important to accurately identify and remove overlaps between lines of image data.
Patent document 2 discloses a fingerprint reading apparatus including two line sensors. The disclosed finger print reading apparatus obtains the movement speed of a finger using the two line sensors, detects distortion of a fingerprint image from the movement speed, and removes overlaps between lines of image data based on the detected distortion.
[Patent document 1] Japanese Patent Application Publication No. 10-91769
[Patent document 2] Japanese Patent No. 3709344
One disadvantage of the configuration disclosed in patent document 2 is that it requires a rather large storage area to store lines of image data and therefore is not suitable for a small embedded system.